Drill rigs can be large and complicated. For the transportation of rigs used to drill deep wells, several days may be necessary to load the rig and many large trucks may be needed to move it. These large drill rigs, however, are not necessary for medium or shallow depth well drilling, say in the range of 5000 ft. maximum depth. For wells of such depth, a rig of more simple design can be used which results in a lighter, more mobile unit.
Those rigs classified as medium depth mobile drill rigs utilize one or more hydraulic cylinders to raise and lower the drilling pipe and may be divided into two basic design types. The most common design type utilizes flexible steel chains or cables operating over sprockets, rollers or sheaves to connect the movable part of the hydraulic cylinders to a drill head guided by the mast of the rig. The drill head is thus indirectly moved up or down in a vertical track by the extension or retraction forces provided by the hydraulic cylinder and chain operable system.
The less common drill design type connects the moving part of the hydraulic cylinder system directly to the drill head or, alternatively, it may use a tube in which the cylinder system is mounted. In these designs, the cylinders are mounted adjacent to the path of movement of the drill head and it is customary to utilize the extension forces of the cylinders to provide the necessary vertical hoisting movement. The cylinder system therefore acts as a column on which is imposed an axial compressive load. Because the load capacity of a column of given length is dependent upon it being constrained in a straight line to prevent buckling, the hoisting capacity and stroke of the rig are dependent on the mounting and guidance of the hydraulic column system.
In these prior direct acting rig designs, the cylinder systems utilized to move the drill head have disadvantages. A first problem relates to the stability of the cylinder system. In one configuration, the piston rod of the lower hydraulic cylinder is connected to the bottom end of the mast. The hydraulic cylinder moves vertically relative to the piston rod and the upper piston rod moves vertically relative to the hydraulic cylinder. With an arrangement of this nature, the hydraulic system column cannot be rigidly connected to the mast at any other point due to the relative motion of the components making up the column. Further, the lower piston rod is necessarily exposed to the elements and various rig operations. Since seals must isolate the rod from the cylinder, naturally resulting mechanical damage, dirt, corrosion, etc., can adversely affect the seal integrity.
A further problem is the unnecessarily large cross-sectional area of the tube. It is advantageous, because the tube is raised and is in the air, to have a minimal cross-sectional tube area to reduce its weight. When the piston rod is rigidly connected to the mast, the centrally located cylinder must be movable relative to the tube and return hydraulic lines must be positioned outside the cylinder. The tube, in turn, must be large enough to accomodate the moving return lines in addition to the hydraulic cylinder.
Yet a further problem relates to the couplings between the mast, piston rod, cylinder and tube. In present rigs, the drill head is connected to the tube but since the tube moves relative to the cylinder, guidance of the cylinder center point is dependent on guidance of the tube. Two concentric supports, each necessarily having an accompanying tolerance, are required. This results in unavoidable tolerance buildup thereby reducing the rigidity and hence the column strength of the hydraulic assembly.